Henerik Kocher's dictionary of Latin proverbs gives the Regimen Sanitatis Salernitanum as the source of the phrase, but I cannot find it in the Latin text.
Another text suitable for framing and hanging in the loo is a little poem composed by the precocious Gargantua (Rabelais, Gargantua and Pantagruel, book I, chapter 13). This translation by J.M. Cohen relies heavily on the Urquhart-Motteux translation:
Shittard,Saint Anthony's Fire is the devastating disease also known as ergotism, caused by the chemical ergot and found in rye infected with the fungus Claviceps purpurea. Rare now, it was once common, and the physician Rabelais was doubtless well familiar with it.
Squittard,
Crackard,
Turdous,
Thy bung
Hath flung
Some dung
On us.
Filthard,
Cackard,
Stinkard,
May you burn with St Anthony's fire
If all
Your foul
Arseholes
Are not well wiped ere you retire.
Chiart,
Foirart,
Petart,
Brenous,
Ton lard
Chappart
S'espart
Sus nous.
Hordous,
Merdous,
Esgous,
Le feu de sainct Antoine te ard!
Sy tous
Tes trous
Esclous
Tu ne torche avant ton depart!
Laughter is also excellent medicine, and I prescribe chapter 13 of the first book of Rabelais' Gargantua and Pantagruel, one of the funniest things I have ever read.